1. Field of the Invention
The present invention relates generally to Faraday rotators, and, more particularly, to the utilization of electrically controlled Faraday rotators in lightwave transmission systems to modulate the polarization of optical signals.
2. Related Art
In many lightwave communication systems them is an increasing desire to provide direct optical amplification, in contrast to prior arrangements which utilized opto-electronic converters and electronic regenerators/repeaters to boost signal power. Some applications include undersea voice and data communication systems, and terrestrial broadband distribution systems.
The attraction to replacing the optoelectronic regenerators in long-range transmission systems with optical amplifiers is the potential of creating a "lightpipe" which is transparent to the bit rate and transmission format. In addition, the use of optical amplifiers enables a transmission system to convey wavelength-division multiplexed signals without the need for multiple repeaters. This elimination of repeaters is significant since the cost of long distance lightwave communication systems is closely related to the distance spanned by the optical fiber transmission medium between repeaters. As this distance increases, the relative cost of a system generally decreases with respect to cost of installation and maintenance.
Long distance lightwave communication systems employ amplifiers periodically along the fiber cable. Each discrete amplifier boosts the optical signal power supplied to the next span of fiber in much the same manner as conventional electronic amplifiers for analog coaxial-cable systems. See Optical Fiber Telecommunications II (S. E. Miller et al., eds., Academic Press 1988). Optical isolators are generally employed with each amplifier to avoid feedback effects.
There is a considerable effort under way to develop rare earth doped fiber amplifiers for use in these systems. Rare earth doped optical amplifying fibers are know to have low cost, low noise properties, a relatively large bandwidth, and minimal crosstalk. In use, rare earth doped optical fiber amplifiers are usually coupled end-to-end with an optical communication fiber and are further coupled (via a wave division multiplexer) to a laser diode pump signal source. The presence of the pump signal (at a particular wavelength, for example, 980 nm or 1480 nm) with the communication signal within the rare earth doped fiber results in optical gain of the communication signal. There exists many arrangements in undersea and terrestrial lightwave transmission systems for providing doped fiber amplification. One such arrangement is disclosed in U.S. Pat. No. 5,042,039 to Edagawa et al. As described therein, an erbium-doped optical fiber is utilized to provide gain to a communication signal by simultaneously passing a "pumping" signal (at an appropriate wavelength) through the erbium-doped medium.
However, there are difficulties associated with transmitting data over long optical fiber spans with cascaded discrete amplifier stages, including chromatic dispersion, polarization dispersion, transmission fiber nonlinearities, optical amplifier nonlinearities, and the accumulation of noise. The evolution of the signal and noise in concatenated optical amplifiers has been discussed in T. Mukai et al., S/N and Error Rate Performance in AlGaAs Semiconductor Laser Preamplifier and Linear Repeater Systems, Quantum Electron QE-18(No. 10):1560-1568 (1982). In practice, amplifier saturation must be considered in view of the competition between the amplified spontaneous emission (ASE) and the optical signal in view of the power available from the optical amplifiers. With concatenated optical amplifiers, this problem becomes especially severe as the ASE, which is characterized as white noise, builds up in successive amplifiers to cause a noise power that becomes progressively higher relative to the single power. In a transmission system this problem is exaggerated by an effect referred to as "hole-burning" where the cumulative gain of the noise, which is at all polarizations, can be higher than that of the signal, which is generally highly polarized. That is, there is a relative depletion of the excited states at the polarization of the optical signal, resulting in a slightly reduced gain in each amplifier for the signal relative to the noise. For example, a typical amplifier in a long-range system has a gain of approximately 10-12 dB. In such a system, if the noise experiences a gain of 10 dB and the signal experiences a gain of 9.90 dB (slight hole-burning), this results in a signal which experiences 10 dB less gain over a system with 100 amplifiers. This has a severe impact on system performance.
What is needed, therefore, is a means for the signal to uniformly utilize all of the polarization states of the fiber amplifier to avoid this cumulative polarization-dependent gain effect in long range lightwave transmission systems having cascaded amplifiers.